Reading this article about a new service that lets you create a virtual card to sign up to free trial services, then auto-cancels at the end of the trial period (after advising you, in case you want to keep it going).
Seems like this would be a good feature for Monzo to explore.
Virtual cards is discussed here:
Make sure you vote for it and add your suggestion in there 
Loads more I could add to this, but I’ve used up all my typing energy explaining it in a topic I started!
Basically, Monzo could make some really good affiliate money out of this, but by legitimising it, NOT by using fake cards. That’s the wrong approach in my opinion. Plus I’m not sure how well they would stand up to an active card check.
Making a system where you decide if you want to continue with a service after a free trial is much better than using a disposable card number, in my head anyway!